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United States Patent |
5,269,190
|
Kramer
,   et al.
|
December 14, 1993
|
Apparatus for the performance of rheological measurements on materials
Abstract
A universal measuring apparatus for the determination of rheological
properties of viscous, viscoelastic and purely elastic materials measures
said properties solely by axial movement of the mechanical measuring
components of the apparatus ("axial rheometer"). the apparatus comprises a
stiff metal frame (1, 2, 3) of which one horizontal part (3) is used as a
mount for a force transducer (9) with little or practically no deflection
at full scale load and on which an exchangeable piston-shaped holder (10)
is mounted axially while the opposing part of the frame (1) forms a mount
for a displacement transducer (4) of the micropositioner type which acts
directly upon an axially moveable piston-shaped holder (6, 8), the
micropositioner being able to cause an axial displacement and to
continuously determine the axial position of the moveable piston-shaped
holder (6, 8) with an accuracy of approximately 0.1 .mu.m, thereby
producing a deformation in a sample (11) positioned between the sample
holders (8, 10).
Inventors:
|
Kramer; Ole (Tornevangsvej 34F, DK-3460 Birkerod, DK);
Winther; Grethe (Ved Ungdomsboligerne 24.1.th, DK-2820 Gentofte, DK)
|
Appl. No.:
|
720423 |
Filed:
|
June 28, 1991 |
PCT Filed:
|
January 12, 1990
|
PCT NO:
|
PCT/DK90/00010
|
371 Date:
|
January 16, 1991
|
102(e) Date:
|
January 16, 1991
|
PCT PUB.NO.:
|
WO90/08309 |
PCT PUB. Date:
|
July 26, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
73/822 |
Intern'l Class: |
G01N 011/00 |
Field of Search: |
73/818,821,822,823,825,59,58,81
|
References Cited
U.S. Patent Documents
2325027 | Jul., 1943 | Anway | 73/823.
|
4383450 | May., 1983 | Pringiers et al. | 73/81.
|
4848141 | Jul., 1989 | Oliver et al. | 73/81.
|
Foreign Patent Documents |
2522362 | Aug., 1979 | DE.
| |
2935118 | Mar., 1980 | DE.
| |
3240666 | May., 1984 | DE.
| |
161944 | Jul., 1989 | NO.
| |
Primary Examiner: Myracle; Jerry W.
Attorney, Agent or Firm: Watson, Cole, Grindle & Watson
Claims
We claim:
1. Universal measuring apparatus for making rheological measurements on
viscous, viscoelastic and purely elastic materials, the apparatus
functioning exclusively by axial movement of its mechanical measuring
components and having a frame (1, 2, 3) of metal or the like, an
upper/lower horizontal part of the frame (1) being used as a mount for a
displacement transducer (4) for controlling in an axial direction a
downward/upward pointing, exchangeable, piston-shaped holder (6, 8) and an
opposing horizontal part of the frame (3) being used as a mount for a
force transducer (9) to be actuated in the axial direction and on the
upper/lower side of which force transducer an upward/downward pointing
exchangeable piston-shaped holder (10) is mounted, the holders further
having a shape allowing to hold between them a sample (11) whose
rheological properties are to be measured, and wherein
the displacement transducer (4) is a high precision encoder mike with a
resolution of about 0.1 .mu.m and a total travel of up to 50 mm,
the force transducer (9) measures forces with a relative accuracy of up to
10.sup.-7 and in combination with the displacement transducer allows
determination of the total deflection of the apparatus with an accuracy
better than 0.2 .mu.m for the full load range of the apparatus, and
the sample holders (8, 10) each have forms according to the measured
material being purely elastic, viscous or visco-elastic.
2. Universal measuring apparatus according to claim 1, wherein the
displacement transducer (4) is a high precision encoder mike in series
with a micropositioner of the integral capacitance piezo transistor type
which has a resolution of about 1 nm.
3. Universal measuring apparatus according to claim 1 for the performance
of stress relaxation and creep measurements on materials, wherein it has a
manually adjustable eccentric (21-24) for giving the sample a quick axial
displacement the magnitude of which is determined accurately with the
displacement transducer whereupon the force relaxation is measured by
means of the force transducer (9) or the creep in the sample is measured
by means of the micropositioner (4).
4. Universal measuring apparatus according to claim 2, wherein the
eccentric mechanism (21-24) which is positioned in the upper part of the
frame (1') comprises a circular disc (21) fastened eccentrically to a
horizontal shaft which is provided with a handle allowing the eccentric
disc to be turned relative to a reference position given by a stop (23),
and in that a smaller frame (22) slidable in the vertical direction is
mounted in the upper part of the frame (1') and in which smaller frame the
micropositioner (4') is fastened, the smaller frame (22) being able to
move up and down under the action of the eccentric disc.
5. Universal measuring apparatus with sample holders (8, 10) according to
claim 1, wherein
the holders are two horizontal parallel plates, or
the holders are two vertical, coaxial cylinders, or
the holders have vertical plates of which two outer plates individually or
combined into one unit rest on the lower holder while a middle plate is
mounted on the upper holder for an axial movement between the two plates
of the lower holder, or
the lower holder at its ends or circumference is equipped with edge-like,
upwards pointing supports for carrying a horizontally place rod-like
member, the bending force of which is to be measured, and the upper holder
being a vertical rod or piston with a tapering lower end for yielding a
mid-point pressure on the horizontal member.
6. Universal measuring apparatus according to claim 1, including a spring
mechanism (20) for providing a force which acts on the upper holder (6, 8)
so that the holder is in close contact with the lower end of the
micropositioner (4) at all times, and so that play is counter-acted during
the axial up-and-down movement of the holder.
7. Universal measuring apparatus according to claim 1, wherein the force
transducer (9) consists of a single weighing cell and that the lower
sample holder is mounted in such a way that the sample in question is
always placed in the vertical centerline of this weighing cell.
8. A universal measuring apparatus for making rheologic measurements on
viscous, viscoelastic and purely elastic material samples, said apparatus
comprising:
a frame which includes a lower part and a detachable upper part and
provides first and second horizontal members,
a force transducer mounted on said first horizontal member for measuring a
force along an axis,
a first piston-shaped sample holder mounted on said force transducer,
a displaceable transducer mounted on said second horizontal member and
displaceable along said axis,
a second piston-shaped sample holder positioned between said displaceable
transducer and said first piston-shaped sample holder to provide for a
material sample to be positioned and retained between said first and
second holders,
said displaceable transducer consisting of a high precision encoder mike
with a resolution of about 0.1 .mu.m and a total travel of up to 50 mm,
and
said force transducer measuring force with a relative accuracy of up to
10.sup.-7 and, together with said displacement transducer, providing a
determination of the total deflection of the apparatus with an accuracy of
better than 0.2 .mu.m over a full load range of said apparatus.
Description
The present invention relates to an apparatus for the performance of
rheological measurements on viscous, viscoelastic and purely elastic
materials for the determination of the rheological properties of such
materials. Such instruments are generally designated as rheometers, most
of which belong to two main categories: Rotational rheometers and axial
rheometers. Rotational rheometers measure the rheological properties of
materials through a rotational action, which in some cases may be combined
with a normal force measurement in the axial direction of the apparatus,
while axial rheometers measure the properties exclusively through axial
motion of the mechanical measuring components.
The rheological properties of purely elastic materials are usually given in
terms of the modulus (stiffness) or, alternatively, in terms of the
compliance. In case of simple liquids, the rheological properties are
given in terms of the viscosity.
Polymeric materials are viscoelastic, i.e., they exhibit properties which
are characteristic of liquids and solids, both. This means that time and
shear rate together play important roles in the measurements and in the
reporting of the rheological properties of polymeric materials.
Polymeric materials exhibit a stiffness which decreases with time at fixed
deformation. This property is given in terms of the Stress Relaxation
Modulus, G(t), which therefore is a decreasing function. Correspondingly,
it has been found that polymeric materials creep under a fixed load, i.e.,
the deformation increases with time. This is given in terms of the Creep
Compliance, J(t), which therefore is an increasing function. These
properties may also be given in terms of the dynamic-mechanical properties
for which the properties are given as functions of the angular frequency
.omega.. The stiffness properties are given by the Storage Modulus
G'(.omega.) and the Loss Modulus G"(.omega.) while the creep properties
are given by the Storage Compliance J'(.omega.) and the Loss Compliance
J"(.omega.). G"(.omega.) and J"(.omega.) are measures of the viscous
properties of the material.
The properties are linear at small deformations and small rates of
deformation, i.e., the moduli are independent of the magnitude of the
deformation and the viscosity is independent of the shear rate, thus
making it possible to calculate one type of property from another type of
property. This is, however, not the case for large deformations and/or
high rates of deformation.
It is common to distinguish between the following main types of rheological
measurements:
Stress-Strain measurements which for simple sample geometries may be used
to calculate a modulus. In some cases a flow limit is observed. This may
be given in terms of modulus and deformation at the onset of flow. Some
rheometers may further allow determination of stress and strain at the
break point for solid-like materials. Several test geometries for the
performance of stress-strain measurements are being used.
Shear viscosity measurements which usually are perfomed at varying shear
rates. This type of measurement is typically performed by shearing the
liquid between two plates which rotate relative to each other (rotational
viscometry) or by applying a pressure to force the liquid through a
capillary (capillary viscometry).
Elongational viscosity measurements which usually are performed by
stretching of a highly viscous cylinder which consequently decreases in
diameter during stretching.
Stress relaxation measurements during which the decrease in stress is
measured as a function of time at a maintained deformation. Several test
geometries are being used.
Creep measurements during which the increasing deformation is measured as a
function of time at a maintained load. Several test geometries are being
used.
Dynamic-mechanical measurements during which the properties are measured as
a function of frequency. Several test geometries are being used.
It is often important to be able to determine the properties of viscous,
viscoelastic as well as purely elastic materials on very small quantities
of sample, e.g., a few grams or less. With the hitherto known techniques
this has only been possible using rotational rheological instruments.
So-called Universal Testing Machines of the axial rheometer type require,
however, fairly large, solid test pieces and determine polymer melt
viscosity by the capillary method, a method which requires rather large
quantities of sample. Universal testing machines of the axial rheometer
type are not suitable for the determination of power low polymer melt
viscosity by the so-called "squeezing flow" method with a constant moving
plate velocity to be described below. The accuracy in the axial movement
and in the determination of plate distance being insufficient in these
machines in that it is an important feature of the "squeezing flow" method
according to the present invention that as little as 30 mg of sample may
be required for the determination of polymer melt viscosity.
The properties of viscous materials are therefore usually determined using
rotational rheometers while the mechanically simpler Universal Testing
Machines of the axial rheometer type are used extensively for measurements
on solid materials where large test pieces can be used. Both rotational
rheometers and axial rheometers may be used for the determination of the
properties of viscoelastic materials, although rotational instruments
usually are preferred for such measurements.
The design and use of rotational and axial rheometers are known for example
from U.S. Pat. Nos. 3,933,032, 4,074,569 and 4,601,195 and from German
Patent Publications Nos. 2,522,362, 2,935,118 and 3,240,666. Further,
Norwegian published application No. 161,944 discloses an axial compression
viscometer for measuring the viscosity of visco and viscoelastic
materials, the upper sample holder of which viscometer being a plate
coupled to a constant speed, hydraulic or pneumatic cylinder controlled by
a position indicator giving the upper plate position with an accuracy 0.01
mm (10 .mu.m), while the lower sample holder is mounted on three weighing
cells controlled by a microprocessor with an accuracy of 10 grs of the
weight of the sample in question. So far, the construction of rheometers
has been based on a principle according to which one unit--the
actuator--has produced a deformation in the sample while a different,
separate unit--e.g. based on a strain-gauge or a linear variable
differential transformer--measures the magnitude of the deformation or
displacement. In practice this has meant that the best obtainable accuracy
of the axial displacement has been about 1 .mu.m. Another disadvantage of
the hitherto known technique is that in order to be able to perform all
the main types of rheological measurements, a laboratory has had to invest
in both rotational and axial rheometers of which the universal rotational
rheometers are more complicated in design, function and operation.
It is therefore an object of the present invention to provide a rheometer
which operates according to the simple principle of axial movement and
which is truly universal in the sense that it can perform all the
above-mentioned main types of rheological measurements: Stress-strain,
viscosity, stress relaxation, creep and dynamic-mechanical measurements,
and which can perform these measurements on very small samples with
different sample geometries and with an accuracy which is considerably
better than .+-.1 .mu.m for the axial displacement.
The object is obtained through a universal apparatus according to the
present invention for measuring the properties of purely elastic, viscous
and viscoelastic materials, and which exclusively functions by axial
displacements of the sample holders of the instrument ("axial rheometer"),
and which consists of a frame made from metal or a similar material, and
where the upper part of the frame is used as a mount for a displacement
transducer which pushes directly on a downward pointing, exchangeable
piston-shaped holder which travels in the axial direction, while the lower
part of the frame serves as a mount for a force transducer or digital
balance which operates in the axial direction and on whose top side is
mounted an upward facing, exchangeable piston-shaped holder, and where the
holders are shaped in such a way that between them they can hold a sample
whose rheological properties are to be determined, and it is a
characteristic feature of the instrument, that the displacement transducer
is a micropositioning device in the form of a high precision encoder mike
and/or a micropositioner of the integral capacitance type which
practically continuously both produces and with an accuracy of up to 1 nm
determines the variable distance between the sample holders while the
latter are producing a deformation in the material sample during a
vertical displacement of up to 50 mm, and that the force transducer is
capable of measuring forces with a relative accuracy of up to 10.sup.-7 of
full scale, typically 40 N with an accuracy of about 0.1 mN in the whole
range, and of determining the total deflection or compliance of the
apparatus with an accuracy better than 0.2 .mu.m, the deflection being
less than 20 .mu.m.
This determination of the apparatus deflection or compliance, i.e., the
combined effects of bending, stretching and compression of the various
apparatus components under load, is made with the aforementioned accuracy
within the also aforementioned load of 40 N in letting an upper sample
holder with a spherically shaped end push directly on the flat bottom of
the lower sample holder, the micropositioner thereby registering the
compliance travel of the apparatus and the force transducer registering
the corresponding force.
It is obvious for a person skilled in the art that although the invention
for reasons of simplicity are explained with the force transducer being
mounted in the lower part of the frame and the displacement transducer in
the upper part of the frame the inverted mounting of the transducers might
just as well be used if appropriate.
It is a further characteristic of the apparatus that its construction also
allows for accurate rheological measurements to be performed in the linear
viscoelastic range with a total travel of the micropositioner of down to
0.01 mm (10 .mu.m) from the first to the last measuring point, that it
allows for the determination of physical dimensions and mechanical
properties of both stiff and soft objects, including thickness and
stiffness of soft objects such as soft contact lenses with an accuracy in
thickness better than 1 .mu.m, and that it allows for measurements of
viscosity of liquids with an intermediate and high viscosity by squeezing
the liquid between two parallel plates ("squeezing flow"). For this
purpose the sample holders according to the invention are two horizontal
plates, one of which by the micropositioner is forced towards the other
with steady and constant velocity over an interval a where 0<a<50 mm, and
where the distance between the plates continuously is determined with an
accuracy better than 0.2 .mu.m.
It is still a further characteristic of the apparatus according to the
invention that the micropositioner via the sample holders can be
controlled to produce deformations which may vary in an
increasing/decreasing manner either continuously or stepwise. The
micropositioner may be a high precision micrometer screw driven by a DC
motor with angular coding (e.g., of the type ORIEL ENCODER MIKE.RTM.) and
with a resolution of better than 0.1 .mu.m, or, for measurements requiring
a very small total travel, the micropositioner may further be of the
integral-capacitance piezo translator type with a resolution of better
than 1 nm.
It is yet a further characteristic of the apparatus that a manually
controlled eccentric is used to produce a quick, predetermined deformation
in connection with stress relaxation and creep measurements, that the
exchangeable sample holders, between which a sample is positioned, are
appropriately shaped for alternative performances of a number of different
tests of the samples and for a quick exchange of samples, that a spring
mechanism is used for forcing the upper holder firmly against the lower
end of the micropositioner in order to reduce play during the up and down
movement of the holder, and that the force transducer may be a simple
weighing cell allowing for an immediate centering of the sample on the
lower holder in the vertical centerline of the cell.
The apparatus is further provided with well-known technical means for
interacting with computer equipment.
The novel features of the invention are thus that the apparatus can be used
as a universal rheometer in the true sense of this expression with small
physical dimensions similar to those of small rotational rheometers while
having the relatively simple mechanical construction of an axial rheometer
as described above, thus constituting a far more cost-effective rheometer
than hitherto possible, that the apparatus according to the invention
combines a small and very stiff frame with a surprising new application of
a micropositioner of the high precision encoder mike type and/or of the
integral capacitance type hitherto only known from an entirely different
technical area (electro-optics)--a combination which now makes it possible
to perform rheological measurements on very small samples and to use the
rheometer according to the invention for the determination of polymer melt
viscosity on very small samples by the squeezing flow method with constant
velocity of the moving plate in which case the plate distance must be
known with an accuracy of better than 1 .mu.m at all loads, that the
micropositioner is used both for producing as well as accurately
determining the variable distance between the sample holders of the
rheometer whereby the usual application of a separate displacement
transducer, for example an LVDT (Linear Variable Differential
Transformer), for the determination of the deformation in a sample is
avoided, and that the apparatus can measure deformational changes in
materials for the total travel of the displacement transducer (up to 50
mm) with considerably greater accuracy (up to 1 nm) than possible with
Universal Testing Machines of the axial type.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail in the following with reference
to the drawings which schematically and by way of example show an
embodiment of the invention in that:
FIG. 1 is a front view of the rheometer in a version without an eccentric.
FIG. 2 shows the upper part of a frame with an eccentric for making quick
step strains.
FIG. 3 shows schematically the rheometer connected to a personal computer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, the apparatus shown comprises a solid aluminum
frame with sides 2, a bottom plate 3 and a detachable upper part 1, which
is fastened to the sides 2 with threaded bolts 18 with knurled,
disc-shaped handles. The bottom plate 3 rests on legs 16 and is provided
with three adjustment screws 14 with knurled disc-shaped handles 15 for
adjustment of parallelism of the sample holders. The screws 14 carry a
mounting plate 13 on which a force transducer is mounted with screws 5.
The preferred force transducer is a digital balance of the type "Mettler PM
4000" with a digital display 12. An exchangeable piston shaped holder 10
is mounted on the digital balance axially in relation to the mounting
plate 13. The holder 10 may be provided with a flat surface on which a
solid or a highly viscous sample 11 maybe positioned directly or on a
metal plate or in a container or the like (10a in FIG. 3), and the surface
may further be provided with a peripheral, vertical rim to ensure
containment of liquid samples (10b in FIG. 3). Diametrically opposed
vertical, upward directed knife-edged rims (10b in FIG. 3) may serve as
supports for stiff, solid samples to be tested in a three-point bending
procedure. A piston-shaped sample holder 8 is mounted in the upper part of
the frame I axially in relation to the sample holder 10 and able to move
freely in the axial direction. The upper-end 6 of the holder 8 rests
against the lower-end of the micropositioner 4 which is fastened to frame
1 by means of a nut 7. The micropositioner 4 is surrounded by a protective
guard 17. A coil spring 20 forces the flat end of the piston 6 against the
movable end of the micropositioner 4 under a load of typically 5-10 N in
order to reduce play during the up and down movements of the piston.
Cut-outs 19 in the upper part of the frame 1 also allow easy servicing of
the micropositioner 4 and easy exchange of upper sample holder 8.
In FIG. 2 is shown the upper part of the frame 1' of another version of the
apparatus. In this case an eccentric mechanism 21-24 is built into the
frame to produce fast step displacements, e.g. for the performance of fast
step strains in samples. A micropositioner 4' is mounted in a moveable,
smaller frame 22 which slides in an opening in the frame part 1'. A
circular disc 21 has been mounted eccentrically on a horizontal shaft in
the upper part of the frame 1' above the smaller movable frame 22. A
handle 24 is fastened to the horizontal shaft, allowing the eccentric to
be turned manually. The handle 24 rests in its starting position against a
stop 23 and the center of the eccentric is in this position 1 mm below the
center of the horizontal shaft. Thus the eccentric in this version allows
for a vertical displacement of the smaller frame 22 of up to 2 mm by a
rotation of 180 degrees of the eccentric.
The smaller frame 22 and thereby the micropositioner 4' moves vertically in
the upward direction under the action of coil springs 20' when the
eccentric 21 is turned counter-clockwise. The magnitude of the vertical
displacement may be determined by the micropositioner with an accuracy of
0.1 .mu.m. The smaller frame 22 and thereby the micropositioner may be
displaced the same distance in the downward direction in less than 0.1 s
when the eccentric subsequently is turned in the clockwise direction until
the handle 24 hits the-stop 23. In this manner it is possible to perform a
fast and predetermined step deformation of a sample which is positioned
between sample holders 8 and 10 as shown in FIG. 1.
In the version of the apparatus as shown in FIG. 3, the apparatus is
coupled to a control and data acquisition system consisting of a
microcomputer 25, which controls the micropositioner 4' via a cable 28,
and a personal computer 26 which is coupled to the microcomputer 25 and to
the force transducer 9 with cables 29 and 30. The computer has a monitor
27 and a key board 31. The microcomputer 25 may be programmed in a manner
which allows for changes in velocity of the micropositioner 4' without
intermediate complete stops. This is of importance for making
dynamic-mechanical measurements on liquids.
When using the complete set-up as shown in FIG. 3, measurements are made by
positioning the sample, whose rheological properties are to be determined,
between the sample holders of the axial rheometer. The micrometer of the
micropositioner is then started whereby the micropositioner moves the
upper sample holder in the desired direction so that a deformation is
produced in the sample. For simple sample geometries, the rheological
properties may now be calculated on the personal computer from the force
measured by the force transducer ; combined with the also measured
corresponding distance between the sample holders. In a preferred
embodiment of the rheometer, the micropositioner is driven by a DC motor
with angular coding and the micropositioner is controlled by an "Encoder
Mike Controller", the micropositioner thereby not only moving the upper
sample holder but also registering the position relative to a reference
position by means of the "Encoder Mike Controller". The distance between
the plates is therefore continuously known, which means that the
deformation in the sample may be calculated for simple geometries.
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